The most mysterious planets in the known universe are all pretty strange planets: from planet Methuselah, one of the oldest known exoplanets in the universe, to the Hell planet that reaches up to 4700 degrees Fahrenheit on one side and hundreds of degrees below 0 on the other, or TrES-2b which is the darkest planet in existence, these are the weirdest planets in space. WatchMojo counts down ten of the most mysterious planets we know about.